Sun Was Weakest In 2019, Solar Storms Were Smaller In Last Decade, Indian Scientists' Study Finds
Researchers from the Indian Institute of Astrophysics (IIA) have shown that the average radial size of Coronal Mass Ejections (CMEs) occurring in solar cycle 24 was less than that in solar cycle 23.
New Delhi: Researchers from the Indian Institute of Astrophysics (IIA) have shown that the average radial size of Coronal Mass Ejections (CMEs) occurring in the solar cycle of the last decades is only two-thirds of its value in the previous solar cycle. CMEs are episodic expulsions of huge magnetised plasma from the Sun out into space.
Solar cycle 24, which occured from 2008 to 2019, was much quiter than the solar cycle 23 between 1996 and 2007. The study, led by Dr. Wageesh Mishra, was recently published in the journal, Frontiers in Astronomy and Space Sciences. The researchers studied the Earth-directed CMEs and interplanetary counterparts of CMEs (ICMEs) that caused geomagnetic storms.
The intensity of solar activity varies in 11-year long period cycles known as solar cycles. The Sun was weakest in 2019 during the last 100 years, the study said. The average radial size of CMEs, and the mass, size, and internal pressure of explosive phenomena associated with the Sun had decreased significantly during Cycle 24. This was contrary to the expectation that decrease of pressure in the interplanetary medium will be accompanied with increase in radial size of CMEs.
CMEs cause major disturbances in the Earth's atmosphere, and affect the near-Earth space environment by disturbing the orbit of satellites in low-Earth orbits, Global Positioning Signals (GPS), long-distance radio communications, and power grids.
Astronomers expected the effect of weakening of the solar cycle to be reflected in the CMEs because solar activities propagate in interplanetary space. In order to investigate the differences, the scientists observed the radial extent of the CMEs when they reached Earth, during both the solar cycles.
Average Radial Size Of CMEs In Cycle 24 Decreased
The fact that the average radial size of the CMEs in Solar Cycle 24 was two-thirds the average radial size of the CMEs in Cycle 23 was astonishing to the scientists because this was different from usual occurrences. Reduced ambient pressure are expected to cause CMEs to expand into an interplanetary space to a significantly larger size, and result in a large radial size.
Dr Mishra explained that the reduced pressure in the interplanetary space in Cycle 24 is compensated by a reduced magnetic content inside the CMEs, and this is what did not allow the CMEs to expand enough in the later phase of their propagation, according to a statement by the Union Ministry of Science and Technology. Also, reduced geomagnetic disturbances in Cyle 24 compared to Cycle 23 is attributed to the lack of stronger and bigger CMEs arriving at Earth during the latest solar cycle, the study said.
It also found that the gas pressure in the interplanetary space in Cycle 24 was only 40 per cent of the pressure in Cycle 23. Also, the Sun was losing its mass through episodic ejections at a rate which was 15 per cent less in Cycle 24 than in Cycle 23. The rate of loss of quasi-steady (situation changing slowly enough to be considered constant) matter by the Sun was 10 per cent lower in Cycle 24 than in Cycle 23.
To conduct the study, the researchers used publicly available observations from the Solar and Heliospheric Observatory (SOHO) and Advanced Composition Explorer (ACE), which are missions launched by NASA in 1995 and 1997 respectively.
The researchers considered the expansion speeds of CMEs close to the Sun and at Earth, in the study. The change of volume of an expanding substance divided by the time required for expansion is known as expansion speed. The difference in total pressure between the CMEs and the ambient space around them primarity governs the expansion history of CMEs.
The scientists noted in the study that the evolution of the radial sizes and the expansion behaviour of CMEs can be better understood by observing CMEs at different distances from the Sun.
The Aditya L-1 mission, which is a planned coronagraphy spacecraft to study solar atmosphere, will be launched by the Indian Space Research Organisation using the Polar Satellite Launch Vehicle (PSLV) in 2022. It is ISRO's second space-based astronomy mission after AstroSat, which was launched in 2015.
The Parker Solar Probe, which touched recently touched the solar corona, will also help scientists observe CMEs at different distances from the Sun, the authors said in the study.
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